The present invention relates generally to a self-monitor system for checking components and/or elements of a digital control system for an automotive vehicle. More particularly, the invention relates to a self-monitor system performing a checking operation for the digital control system in a cold start, which is insensitive to temporary drops in battery voltage during engine cranking.
Recently, various digital control systems have been used in automotive vehicles for controlling various vehicle components, such as engines, automatic power transmissions, brake systems and air conditioners. Usually, such digital control systems include a self-monitor which detects and announces errors in control operation due to failure of one or more components and/or elements, such as sensors, actuators and/or indicators. Some of the self-monitor systems operate in response to an initial reset or initialization of the digital control system to perform checking or testing operations upon initialization in response to starting of the power supply, i.e., during a so-called "cold start".
As is well known, it takes a great deal of power to drive a starter motor of an internal combustion engine to start the engine. This engine start-up operation is always a heavy load on the vehicle battery and brings about a battery voltage drop. Therefore, it is possible that battery voltage will temporarily drop below an allowable range. Battery voltage can be returned to normal level by recharging it with electric power from a generator associated with the engine. A short period of time is required for the battery voltage to recover to its normal level. If the self-monitor system is active during this period of battery voltage recovery, the battery may be judged to be no good. In addition, when the sensors, actuators and indicators are checked by applying dummy loads thereto, insufficient dummy loads may be applied to the sensor, actuators and indicators. Some of the elements to be checked may not fully respond to the dummy loads and so be judged to be no good. As a result, despite normal operation of the battery and/or elements, malfunction may be indicated.
If self-monitor operations can be performed after the period necessary for recovery of the battery voltage expires, the possibility of erroneous judgement will be minimized or avoided.